1. Field of the Invention
The present invention relates to dosimeters and particularly to a new type of rapid-readout, thermoluminescent dosimeter for the remote monitoring of radiation sources, such as ultraviolet, x-ray or gamma radiation, using a radiation-sensitive glass material (dosimeter) in conjunction with fiber optic components.
2. Description of the Related Art
Thermoluminescent (TL) materials have been used for many years to monitor radiation exposure levels. These dosimeters measure the accumulated radiation exposure over a period of time, ranging from minutes, to days to years. Materials such as metal-ion-activated Lithium Fluoride (LiF), or Calcium Fluoride (CaF.sub.2) are commonly used in "film badges" to monitor personnel exposure to radiation. These materials are generally prepared from powders that are pressed into opaque white pellets. When exposed to ionizing radiation, such as deep ultraviolet, x-ray or gamma radiation, free electrons are generated and are trapped in the material. The electrons remain trapped until a source of heat is applied to the material to stimulate the release of the electrons. The electrons recombine with an ion in the material resulting in the emission of light. The amount of light emitted is proportional to the amount of radiation exposure.
Thermoluminescent dosimetry (TLD) materials that are used in practice are generally limited in size because of the high degree of light scattering. Only light generated near the surface can be effectively used to measure radiation dosages. As a result, commonly used commercial dosimetry materials have dimensions of approximately 2 mm.times.2 mm.times.0.2 mm. This small size limits the dynamic range and ultimate sensitivity of the material.
The traditional approach to TLD involves the collection of the dosimeter material from a film badge or other monitoring package and placement of the material inside a machine that heats the sample at a controlled rate and monitors the light emission as a function of temperature.
Glass materials have been studied for radiation dosimetry measurements. With some glasses, radiation exposure leads to darkening of the glass and the degree of darkening is used as a measure of the radiation dose. Thermoluminescent glasses have also been reported. The effectiveness of these glasses for TLD applications has been limited for a number of reasons, including low readout temperatures, low sensitivity compared to crystalline phosphors and low saturation doses.
Fiber optic TLD systems have also been described. One system utilizes traditional TL phosphors attached to the end of a 0.6 mm diameter optical fiber. An absorbing material is applied to one surface of the phosphor and a diode laser is used to heat the absorber which in turn heats the TL material by diffusive heating. This system is described as a remote fiber optic laser TLD system. The performance of the system is limited in several ways. First, the TL material must be very thin, approximately 0.1 mm, to allow the laser heating source to be transmitted through the TL material to the absorber material. As a consequence, in order to attain sufficient TL sensitivity, the diameter of the TL material must be fairly large. The diameter of the optical fiber must also be large to match the size of the TL dosimeter. Applications that involve in vivo monitoring of radiation exposure in the human body via fiber catheterization can be improved if smaller fibers can be used.
A laser heating method has been described for the heating of TL materials stacked in layers. In this study, a CO.sub.2 laser was used as the heat source. This does provide for rapid, efficient heating but is impractical for fiber optic applications because ordinary optical fibers are not transparent to CO.sub.2 laser wavelengths, and specialty, CO.sub.2 -transmitting fibers are of limited utility, having large diameters(0.7 to 2 mm diameter), limited transparency for visible light wavelengths that correspond to the thermoluminescence and are very expensive (one meter of fiber costs approximately $1000, 10 meters cost $5500).